Thermal energy

Thermal energy is a word used in science and engineering that can mean a few different things.

Sometimes it means the energy stored inside objects or radiation, called internal energy. This is not the same as the energy that holds everything together, known as potential energy.

Thermal energy can also mean heat. Heat is the energy that moves between objects and their surroundings, but not by working or moving matter.

Thermal energy may also describe energy linked to temperature. This energy depends on how hot or cold something is and a number called the Boltzmann constant.

Because “thermal energy” can be confusing, some scientists, like Mark Zemansky, say it’s better to use words like “internal energy” and “heat.” But some books still use the term thermal energy.

Relation between heat and internal energy

In thermodynamics, heat is a kind of energy that moves between objects. This can happen in many ways, like feeling warmth from the sun or a metal spoon getting hot in soup. Heat is not something an object "has" — it’s about energy moving from one place to another.

Internal energy belongs to one object or system. It is a property of that thing’s current state, like how much energy its atoms have right now. Heat and work are ways energy can move, but internal energy is about where the energy is at any moment.

Macroscopic thermal energy

Thermal energy is the energy inside an object. It comes from the tiny movements of molecules and the energy stored in the bonds between them. When we talk about thermal energy, we are often talking about the energy stored inside something because of its temperature.

Chemical reactions can change the energy inside an object. For example, when a reaction happens, the energy stored in the bonds of molecules can turn into other kinds of energy. We might say the chemical energy has turned into thermal energy, like when we feel heat from a chemical reaction.

Microscopic thermal energy

Thermal energy is the energy inside tiny parts of matter, like the small pieces that make up a gas. When these pieces move, they have kinetic energy, which is the energy of motion. This movement is what we feel as heat when it moves from one place to another.

In physics, thermal energy can also mean something called "kT." This is a way to measure energy using a special constant and the temperature of the material. It helps scientists understand how temperature affects the energy of tiny particles.

Thermal current density

When there is no movement of matter, "thermal energy" can mean the energy moved by heat flow.